text:
What is the typical pilot to aircraft ratio in military squadrons?
On aviation.SE, we already find this question addressing the pilot to aircraft ratio in airlines. It states that typical big airlines have roughly between 15 and 20 pilots per aircraft.
Military aircraft may require less pilots per aircraft:

airliners are in flight almost all day long every day
a typical airliner flight requires between 2 and 3 pilots (the 3rd one for very long steps or as an instructer)
a typical fighter is a one-seater
a fighter flight less time than an airliner
military cargo may flight more than fighters, but I hardly see a cargo used as much as a passenger aircraft (I may be mistaken)
HALE UAV may require several crew per flights but I don't know how long are those flight neither how many UAV can be operated simultaneously by one crew

When searching the number of each type of pilots (fighters, cargo/airliners, UAV) per air force (whatever the country), it is easy to come across recruiting websites (with requirement to become a military pilot and salary) but I fail to find anything about the number of pilot an air force has.
My question is : What is a typical ratio of pilots to aircraft in a typical military squadron? Does it differ between fighter squadron, transport squadron and UAV squadron?

They typically write the names of the pilot(s)/crew (and ground chief) on the side of the aircraft, so I'd say it's pretty much 1:1 neglecting training aircraft. I know this holds for fighters, I'm not sure about the larger transport/cargo type ones.

@RonBeyer Good point. I forgot that detail (that brings other questions to my mind, I may continue asking new question within the next hours, the time to make basic research)

Which nation? Some nations punch well above their weight when you consider their defence budget, but this same budget means less cash to train up the pilots, so there are less around. This is offset by making pilots highly trained, medically fit with full background checks, and to sign them on for a decade or so to get a return on investment. So there are a lot of variables at play, but if I had to say a number, it's about 1 to 1 in my limited experience.

@Craig As I fail to find information for any country, any nation for which you have figure will be a good one.

@ManuH Defence forces aren't usually in the business of divulging that sort of information. You know, operational security and stuff?

@Craig I'm not sure as it is easy to find crew size of aircraft carrier on wikipedia

@ManuH that aircraft carrier says 600 in the airwing but it takes a lot of ground crew to get a jet flying. I guarantee there would be a bunch of red shirts standing around doing nothing that make up that number.

@Craig my point is that we can easily find some figure for personnel in defence forces, thus I'm not sure "operational security" may not be a good reason for not having those numbers.

As has been said, there is no one exact rule, but a broad estimate can be easily produced.
There are 12,500 active duty pilots in the USAF, and the service counts just over 5,000 manned aircraft. This will give an overall, rough, ballpark figure of 2.5 pilots per aircraft across the entire force.
Counting aircraft types by pilot seats, one can calculate the total number of pilot seats at around 8,350. About a thousand comes from training aircraft, however.
This gives an overall approximate figure of 1.5 pilots per seat, across the entire USAF. Some of the extra 0.5 are senior officers, who won't be the first choice for combat missions, but need to maintain their currency.
This is not distributed equally. As mentioned in the comments above, combat units tend to run ratios <= 1.5. With the recent shortage of pilots, especially ones rated for combat aircraft, it has sometimes fallen below 1:1 in some units (this is rare). For transport and utility aircraft, there tend to be slightly more crews per seat.

1.5:1 is a good number.  That's typical for a Navy squadron.

Also "active duty pilots" may include senior officers and admin officers who maintain their pilot currency but don't expect to fly real operations.

The pilot-to-aircraft ratio depends mostly on the aircraft type the squadron flies. For example, a fighter squadron might have far more aircraft in its fleet than an airlift or reconnaissance squadron. There is no fixed number.

I was expected at least approximation (between X and Y, or around X).

There is no approximation. It could be 1:1, 2:1, 10:1, or 1:10.

For me, your answer give less information than this comment or this answer, thus I think it should be a comment and not an answer, unless you add more precise information that give a good overview of the asked ratio.

I am telling you the most accurate answer, which is that there is no approximation. You would have to specify an aircraft type, a specific country, and then a specific squadron. Then, you would need someone to divulge those numbers to you, which is unlikely.

I fail to understand what information it adds to this comment, which was posted as a comment, not an answer.

Adding on to your answer, many planes in service utilize RIOs as well as pilots


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What is the speed above which painting an aircraft black (or dark) results in lower skin temperature?
The Concorde is painted white for thermal reasons. If I understand correctly, it is done to reflect as much radiation as possible in order to lower tempereture. But when the skin is really hot (SR-71, reentry vehicle such as the Space shuttle,...), it is better to have a black/dark skin so that it radiate internal heat.
In short: when flying supersonic the aircraft should be painted white to reflect incoming radiation, and when flying hypersonic the aircraft should be painted black to radiate internal heat.
My question is: what is the speed threshold below which painting white results in lower temperature and above which painting black results in lower temperature?

the sr-71 was not hypersonic

@ABJX I agree. The SR-71 flew fast enough for black skin to be thermally interesting. Flying hypersonic (i.e. faster) would be fast enough for black skin to be interesting.

wait what? the sr 71 did not glow. if it did, then that would make sense. but do aircraft already glow at mach 5 (hypersonic)?

what is interesting is being black in IR, not visible.......https://aviation.stackexchange.com/questions/51268/why-was-the-concorde-painted-white-and-not-black

This might be more of a question of altitude and the ability to remove heat as a function of air density. The SR-71 has a cruising altitude of ~26km and the Concorde has a crusing altitude of ~18km. Of course a black coating makes it absorb more solar irradiance, but the designers would have taken this into account and ensured that energy balance favors a desirable temperature. The SR-71 had at most two passengers, so comfort cooling can be focused on the cockpit compared to a passenger plane. The Concorde on the other hand had a much larger passenger cabin which would lead to much higher parasitic cooling losses with a black skin.
This chart shows the decrease in density, but whether it's enough to justify a different approach to passive thermal control is hard to say:

The implication is that at high altitudes, heat generated inside the vehicle needs to be removed by a combination of convection and radiation. This is especially important for the Space Shuttle as it needs to maintain temperatures in the lack of atmosphere although the black tiles did not serve as the orbiter's primary radiators. They only needed to insulate the orbiter from aerodynamic forces and the high temperatures. While reentering the atmosphere, the heat comes from air compression in the atmosphere and the black tiles would have helped to re-radiate some of the heat away from the orbiter.

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What is the maximal delay between the publication of an investigation report recomending a study and the beginning of this study?
This question is focused on EASA regulations, mainly because the question arise when reading an investigation report written by the BEA.
Investigation reports results in lots of recommendations to improve safety. Some recommendations involve certification process and are then implemented when certifying new aircraft types, other involve training process and are then implemented by modifying training, other recommend studies.
For example, if I take the AF447 final report, the section 4.3.7 recommends that:

EASA study the relevance of having a dedicated warning [...]

This is not the only study recommended by this report, this is just an example.
On another report named Study on aeroplane state awareness during go-around (2013) (which proves by its title that studies are conducted), the section 8.2 recommends that:

Manufacturers study the feasibility of simplifying the management of the aircraft reconfiguration [...]

Then I imagined this lead to other studies.
In short, reports by investigation entities (NTSB, BEA,...) may recommend studies to be conducted. This leads to other reports, possibly asking for other studies to be conducted. It looks like an endless, yet useful, process.
Are those recommended studies always conducted (maybe this is ensured by an EASA regulation I'm not aware of)? If so, is there a maximal delay after the publication of the recommendation to start the study? Are studies recommended by different types of publication (accident reports, studies report,...) treated differently in respect to those delays?

I can't say with certainty about EASA, but generally these are just recommendations and thus are non-binding.

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How is roll control achieved for the space ship two?
When looking at scaled composite 339 space ship two I fail to see control surfaces on the trailing edge of the wing. For me, it seems that the only moving surface on the wing is the feathering mechanism and that control surfaces are all located on the tail.
My question is: when not in space, how is roll control achieved? does it use horizontal stabilizer in an asymmetric way? Is there a control surface I fail to see?

There is never any control surface on the leading edge of the wing. Only high-lift devices are on the leading edge (slats). The ailerons are normally on the outboard parts of the wing trailing edge, but the only reason they are there is that it is furthest from the longitudinal axis. Now what is furthest from the longitudinal axis here…?

@JanHudec updated

There seems to be some sort of reaction control system integrated into the wing, details seem sparse. see: http://www.parabolicarc.com/2019/08/14/virgin-galactic-completes-wing-for-third-spaceshiptwo/

@NileRiver, the next sentence fragment is the relevant though: “elevon flight controls”.

@Manu H -- At 2:09 in the video (see https://youtu.be/X0WQ0KjScOU?t=129 ) I see no shortage of control surfaces.  Are you familiar with elevons?

@quietflyer are you referring to the surfaces outside the rudders? For me it looks more like a classic tail than a delta wing, and the wing itself is more inside the rudders. Yet I fail to see any control surface movement in your video (given the airspeed, movement should be small), thus I cannot state the elevator is used for roll control.


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Is there a minimal legal ground clearance to use an international airport?
The question arises while I was looking at the evolution of the B737 engines and the lower and lower space available between ground and engines. Then I realized that ground clearance is also low for other small airliners we can find in international airport (e.g. Embraer 175, ATR-42,...).
I imagine ground clearance is designed taking into account tailstrike, FOD, runway irregularities.
My question is: can an aircraft legally be excluded from using an airport because of its ground clearance?
Let's restrict this question to international airports and airliners transporting at least 8 passengers

Are you interested only in airliners?  And what does the ground clearance apply to, engines/propellors or fuselage?

@jamesqf I already restricted the question to airliners transporting at least 8 passengers. I'm interested in both fuselage and engines (i.e. anything that can touch the ground and is not made to)

I don't know whether they've ever been used as airliners, but the Aero Commander family has a pretty low fuselage ground clearance, and some models carry more than 8 passengers: https://en.wikipedia.org/wiki/Aero_Commander_500_family

For propellers, there is a minimum ground clearance according to the EASA Easy Access Rules for Normal, Utility, Aerobatic and Commuter Category Aeroplanes (CS-23):

CS 23.925 Propeller clearance
Propeller clearances with the aeroplane at the most adverse combination of weight and centre of
  gravity and with the propeller in the most adverse pitch position, may not be less than the following:
(a) Ground clearance. There must be a clearance of at least 18 cm (7 in) (for each aeroplane with
  nose wheel landing gear) or 23 cm (9 in) (for each aeroplane with tail wheel landing gear)
  between each propeller and the ground with the landing gear statically deflected and in the
  level, normal take-off, or taxying attitude, whichever is the most critical. In addition, for each
  aeroplane with conventional landing gear struts using fluid or mechanical means for absorbing
  landing shocks, there must be positive clearance between the propeller and the ground in the
  level take-off attitude with the critical tyre completely deflated and the corresponding landing
  gear strut bottomed. Positive clearance for aeroplanes using leaf spring struts is shown with a
  deflection corresponding to 1.5g.
(b) Aft mounted propellers. In addition to the clearance specified in sub-paragraph (a) an aeroplane
  with an aft mounted propeller must be designed such that the propeller will not contact the
  runway surface when the aeroplane is in the maximum pitch attitude attainable during normal
  take-off and landings.

So 18cm seems to be the minimum ground clearance for propellers on most aircraft (e.g. ATR-42).
It is hard to prove a negative, but the section about turbojets and turbofan engines does not list any minimum.


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How are helicopter rotor blades balanced?
Reading this answer, I realized that the balance of a rotor is critical. When one blade is changed, the whole balance of the rotor may be affected. Moreover, I doubt 2 blades are exactly the same. For a car's wheel, we add small weights, but what about helicopter's rotor? Adding pieces to the rotor may affect its aerodynamics.

It's probably not as exacting as you would think. If you look at this question you see that most have a drag hinge to allow for some play. And if you look at this question you will see how that allows for balancing.

Related: Rotor Blade Balancing /// and /// Track and Balance, Robinson R66

Most blades are as close as possible when built at the factory. Yes, it is like a car wheel you use weights to balance. For the Blackhawk helicopter there are inserts on the hub near the blade to add weights. But additionally there are a maximum amount of weight that can be added. If you're unable to balance the main rotor head, sometimes you have to switch blades around to other sides and adjust weights. If it still doesn't balance usually it is due to the installation of the hub to the main transmission then gaps have to be measured.

you may enhance your answer by adding links and references for further readings.

Mass balancing of helicopter blades is pretty much the same as any other rotating component- by addition of masses. Usually, unbalance in the helicopter rotor disc results in lateral vibration- requiring shuffling of masses in rotors for reducing it.
In general, the rotor blades are balanced on ground (statically balanced as a set, usually), so as to reduce the deviations caused during manufacturing process. After installation on the helicopter, the dynamic balancing of the rotor blades is carried out to bring vibrations within a set limit. This is done by addition or removal of masses, usually near the hub area (though this is done near the tip in some helicopters). This should bring the vibration within limits in most cases- if not, this is achieved by replacing the blades individually, or in extreme cases, by changing the whole set.
The masses are usually added near the hub, where the aerodynamic penalty is lesser or adjusted within mass chambers, which are not exposed, thereby reducing the adverse aerodynamic effects.

DO you have any links, references and pictures to illustrate your answer?

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How cost efficient are older jets versus buying a newer jet?
I notice that older private jets are a lot cheaper than newer ones. For example, 2019 Falcon 2000LXS might cost say \$24 million but a 2015 may only cost \$20 million and a new LXS costs \$35 million. Is this price difference new car smell, or are there significant reasons why a newer one is worth so much more?
I ask because I know with smaller aircraft I have flow like Cessna 172s and things like that, the differences between a 1990 and 1970 are more or less trivial (or at least I think they are), so for a smaller aircraft I would definitely buy an older aircraft and save a LOT of money. However, I am wondering if this same logic scales up to jets?
I suspect that it does because I know that a fractional ownership group that I do business with has a policy of always buying only new jets and always selling them as soon as they are 5 years old. I asked them the reason for this policy and did not get any kind of convincing answer, so I got the impression that the policy is purely psychological in that the clients want to fly a "new" aircraft and there seemed to be no practical reason for it. So, if this is the situation that most jet owners (like car owners) dump their jets just because they are "old", then it would seem to make financial sense to only buy older jets. Is this right?

The older an airframe, the more time per year it will spend in maintenance, being unavailable to its customers or in this case fractional-owners. Frequent flying also increases maintenance hours per year. An older jet, flown frequently, will spend the most time getting serviced and thus unavailable.
The depreciation is even worse with airliners - they can drop 10-fold in 10 years, due to the wear and high cost of D-checks. Buying a used jet is like buying a company: you get both the debts and the assets, the debts being wear and maintenance intervals.
Private jets get far fewer flight hours per year than commercial ones, and are subject to less regulation. Trump famously owns a refurbished 757 that was all but written off - private jet service makes a good second life for worn commercial jets.
Fractional ownership is more akin to a charter service than to a true private jet, which won't fly much more often than its owner. These jets get considerable mileage, and will require maintenance. Ideally, you'd want to use up the flight-hours and the years on each part about equally over the life of the airframe. It makes both technical and commercial sense to run a jet hard for 5 years, then sell it off to a private owner, while many parts still have plenty of hours left that might last them a lifetime.
Low-cost carriers, which drive their jets the hardest, also often have a policy of flying only new airplanes. This way, they get the highest availability and predictability, which are important for their schedules. It's also possible, not necessarily for this company, that the jets are leased rather than bought, in which case these years would be a hard limit.
Short version: Older jets are cheaper per airframe. Newer jets are cheaper per flight hour. So you buy new to fly, old to own.

On complex airplanes like jets, it's fairly simple:
Old airplanes tend to bleed you dry on maintenance costs and fuel burn. You can also expect more delays and cancellations due to faults.
New airplanes are the opposite: Low maintenance cost, especially during the warranty period, high reliability, and low fuel burn with up to date engines, at the price of higher finance and insurance costs.
Different operators have different business models. One business case or the other will fit a particular model.
The factors with the most impact are generally fuel prices and interest rates. High prices tend to drive operators toward newer airplanes, and vice versa. On the other hand, an operator may have to go with older airplanes just because they can't finance newer ones.
Newness can be an important factor to the client. Nobody wants to ride around in a beat-up corporate jet. But these airplanes can get a new lease on life with a new interior and paint job and might be a better match for an operation especially when fuel is cheap. A renovated corporate jet will be more or less the same as a new one to the customer that doesn't know any better.
An example of the sensitivity to fuel prices from the regional airline industry. Airlines were starting to unload their CJR200s in the mid 2000s as fuel price hikes resulted in the airplane's relative inefficiency fuel burn wise making difficult to remain profitable. They were switching to newer CRJ700s and 900s. Then fuel prices crashed, and all of a sudden, used 200s could make money again thanks to a low purchase price in spite of the high fuel burn. One regional operator, Skywest airlines, started buying up every CRJ200 it could get its hands on.

For tax purposes,  a private jet used for business purposes can be written down (depreciated) to zero asset value over a certain number of years. The depreciation is deducted from your taxable income, if you have any in that tax year.
This means that depending on the skill level of your tax consultant, you can game the system to your overall benefit as long as there's some depreciated value remaining in the jet as an asset on your books. Once it's zeroed out, then you have to buy a new jet to make the game start over.
I do not know any of the details, but it boils down to a sort of tax break for high-net-worth individuals: private jet sales go up whenever the depreciation schedule on them is accelerated in the tax code.
Funeral home operators do this trick with their hearses, which can be counted on to pump a certain percentage of hearses into the used market every year. Hearse enthusiasts (I used to be one) then shop for cars in this "value dip" zone where the car has gotten retired but is not old enough to have collector value.
Note also that it may happen that the navigation systems in the jet become obsolete when the asset has been fully depreciated. Whoever buys that jet has to do a cockpit upgrade and most likely an annual before they can get it back into the air.  If this coincides with a hot-section teardown on an engine no one uses anymore, then the Lear 23 you just bought for a brief song and an uninspired dance becomes a lawn ornament.


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How do you report reckless pilots?
I have never reported reckless behavior by other pilots to the FAA, but I have thought about it several times. The only problem is that usually I will only know the tail number, not the name of the pilot. Since the owner of the aircraft may be different than the one flying it, how does one go about making sure the complaint is filed against the right person?

The FAA provides an online reporting tool here under "FAA Hotline Web Form" (direct link here) I would say it falls under "aviation concern". In your description if you include tail number and time they should be able to figure out who was flying (or in the cockpit if its a 2 man situation). You can also contact your local FSDO and report the tail number and time of incident. Aircraft owners/renters (the people the tail number will trace back to) will more than likely know who was flying a given tail number at a certain time so as long as you know when it happened you should be ok its not necessarily on you to know who was flying but the tail number and incident time is key.

The safety reporting page has moved to https://faa-aae.my.site.com/s/hotline

Thinking about that link more, it seems odd that we now have to report to a non .gov site, but it's real.

Ok, the following link redirects to the above page. It looks like a more durable option: https://hotline.faa.gov/

Yet another update... The online reporting tool seems to no longer exist - only the local email and phone options. Please share here if you know of a functioning online form.

@GaTechThomas I have updated the link, it appears that the hotline link you found is indeed the legitimate one.

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Is this route of a airforce jet on flightradar faked?
While looking at the flight routes of Indian airforce jets on flightradar, I have noticed that the trainee jets show a very weird flight pattern. Here's an example,

The width of this screenshot is around 20KM. I don't think a jet can take such sharp turns. If so, why are these turns so sharp? Is this data somehow been faked, or is this issue with the radar?

You should click on airplane, and get the information from there. IIRC the last field is important (the "radar"): you can get more information on which kind of data is used. These look like MLAT, so user triangulation, which do no requires precise timing, so you get such weird lines

This is what happens with linear interpolation. But it is more honest, you can really see the data and resolution.

No, that has not been faked.
Flightradar is updated with a low frequency (from that screenshot I'd estimate once per 10 seconds, maybe more). This is done to reduce the computational demands on both the service that sends the radar info to Flightradar, and on the Flightradar servers.
Each dot in the track is one position update, and the website software just draws straight lines between them, which results in sharp corners. The real flight path will have a radius.

authors: Vladimir F Героям слава


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Who decided which airspaces not to fly through and how?
I was looking at some flight paths, and I noticed that while some airspaces are ignored by planes for "obvious" reasons, like the Ukraine - Russian border airspace.
For example, this flight seems to purposely ignore the Ukraine airspace.
But on the other hand, I found some flight which are not ignoring airspaces which should probably be ignored. Like planes flying over Syrian airspace or over Yemen. These places too are war zones and might have militant groups capable of repeating something like MH-17.
This made me wonder as to who decides what airspaces to not fly over, and what are the criteria to do so?

Possible duplicate of Apart Kenya, what are the current dangerous / unsafe airspaces?

Not sure it's a duplicate, because this asks also for the criteria; but definitely related.

One major driving factor is whether a permit is required. Many countries in the world require a permit even for overflight.

(source)
Depending on country to country, airline to country and general relations it may be more advantageous for a carrier to get permits in one country than it would be in a close neighbor. Going a bit out of the way (burning extra fuel ultimately) may be less costly than delays related to permit issues, etc.
In some cases there are also prohibited airspaces within a country. This is simply airspace you cant fly over for regulatory reasons (generally security). While some of these airspaces are small, some areas can be quite large in size.
While increasingly a smaller case not all aircraft are equipped to fly through all airspaces. Different regulating bodies may require different gear on board for a given flight. Aircraft may be prevented from entering a given airspace if not properly equipped or certified.
On any given day routes may be amended for weather or potential weather hazards.
As @mins notes in the comments, this answer covers areas avoided for safety reasons.

In your map, which areas require a permit? What do the different shades of yellow indicate?

@DanieleProcida if you navigate to the source link you can hover over each area and see the requirements.

If there is a large sporting event, or festival, they will limit air traffic around that area. They do this for safety in case an aircraft attempts. A government based aviation regulatory agency would determine where to fly and not fly. If you are flying over a war zone, that would be up to the management of the airline to determine that.
Let me know if you want me to add on.
Hope this helps, Charlie :)

In addition to permits, some countries simply don't allow planes registered in certain other countries to overfly them, period. Conversrly, some countries dont allow their own planes to overfly certain other countries, even if those countries are okay with it. Some countries may allow aircraft to overfly but not land, even in an emergency, which is also problematic since it may be impossible to reach a safe country depending on the emergency. All of those are very common in the Middle East, even absent an actual war. War zones are generally unsafe for anyone to fly in, though that doesn't stop some carriers from doing it anyway--and occasionally getting shot down.
Besides safety, there is also the issue of fees. Many countries gouge airlines to the point it's cheaper to take a longer route around them or at least minimize the time/distance over them. For instance, US domestic routes are adjusted south to only overfly southern Ontario rather than take the great circle route that would spend more (very expensive) time over Canada.


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What is the ratio between time in the air and time on ground for an aircraft?
I suppose it may be different from an airline to another.
Given the maintenance, the time to load/unload the aircraft, the time the aircraft is grounded because there is no flight scheduled (e.g. at night), how many time an aircraft actually spend on ground compared to the time it spend in the air?

What is the significance of the time when an aircraft is sleeping at night? For maintenance, hobbs meter and tach timer are used. Please see this question for details on them.

@Farhan: There are two kinds of time on the ground. Time when the aircraft is in use, but on the ground and time when the aircraft is not in use.

It will vary widely from aircraft to aircraft, but I will try to give some general estimates.
Time on the Ground
Part of what you are referring to what is sometimes called aircraft turn time. It's not really directly related to my calculations for your overall question (though it is certainly a factor for short-haul jets), but in this article, minimum turn times for the Boeing 757 was studied, and it was found that the estimated turn time for a 757-200 was 52.5 minutes, based on actual operations. The 757-300, a stretched model with more seating, would take about 59 minutes.
From this presentation, presented by United Airlines, they show a general guideline MSGT (Minimum Scheduled Ground Time), that is a generalization depending on flight locations, field facilities, and airlines. Some times are:

Boeing 747: 80 minutes
Boeing 777: 70 minutes
Boeing 767: 60 minutes (-300 model)
Boeing 737: 35 minutes (-300 model)
Airbus A320: 40 minutes

This link has a discussion of a number of airliner turn times, however none are from official sources. Most vary between 20-45 minutes, depending on if cleaning is required.
So for a typical airliner we can say it takes about 45 minutes-1 hour for a typical servicing.
Aircraft Lifetime
Now as to how many times they fly, you can find their aircraft design cycle limit (essentially, how many takeoffs and landings it can make in its lifetime) in this Federal Register document (summarized here). Some listed numbers:

Boeing 747: 20,000 cycles
Boeing 777: 40,000 cycles
Boeing 767: 50,000 cycles
Airbus A320: 48,000 cycles (60,000 hours)
Airbus A300: 40,000 cycles

So for a 'typical' airliner we can say it can survive about 40,000 cycles in a lifetime
You can find the average age of airliners in different places online. This site splits up by airline and aircraft, and most airliners have an average age of 12-15 years. But note that this is the active fleet. The average age at retirement for an airliner is now around 24-25 years.
You can find a related Aviation.SE question here
Aircraft Usage
This thread discusses the average cycles and flight hours per day. Long haul aircraft (like the 747, 777, A380) tend to spend more hours in the air per flight, but also have less cycles per day. I can't find official sources, but the thread presents a couple example averages for peak usage rate:

Delta Boeing 757: 9.5 hours/4.8 cycles
United Boeing 747: 13.2 hours/1.6 cycles

(I find these probably accurate, as there have been some publicized accidents that have much higher usage, like this one, which flew 48,740 hours and 39,781 cycles in 15 years - about 7 flights/day; short-haul jets like the one in the article end up with more cycles than long-haul, as I noted above).
It is difficult to find hard numbers, but overall, the average flight length for a short-haul jet is about 2 hours, whereas a long-haul jetliner is around 6-7 hours.

Again, there are a lot of variables in this question, but using the above data, we can estimate the answer to your question, using these estimated figures from above:

40,000 cycles (~20,000 flights)
25 years
2 hours/flight

This means approximately 40,000 flight hours for a typical short-haul jet; over 25 years (219,000 hours), this works out to about 1 hour in flight for every 5 on the ground. Obviously this varies widely, but for a ballpark number, that's the best I can do. The highest usage would be a heavily used long-haul jet used on the longest routes from a busy airline (i.e. trans-Pacific; Middle East to U.S.), whereas the lowest would be a moderate-length flight on a low-volume route. In general, you are probably looking at about 1:3 to 1:5 air:ground time.

1:3 to 1:5? Even the 9.5hours/day example is 1:2.5 only.

@JanHudec - from my research, Delta has one of the highest usages among airlines, due to its many many routes. Plus, as best as I can tell, those numbers are not a lifetime figure, but a in-operation figure, not counting times when it is undergoing maintenance, or when it is being phased out in favor of a newer jet near the end of its life.


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Is the hydraulic fluid artificially colored?
A normal aircraft has multiple hydraulic circuit, identified by colors (e.g. according to the ATSB final report of the QF32 the green hydraulic line was disabled; according to the BEA final report of the AF4590 the Concorde had a yellow, a blue and a green circuit, ...). I understand the need to be equipped of several independant hydraulic system (redundancy) and the need of being able to identify those.
Thanks to the previous example, I figured out the hydraulic circuits are identify by colors.
Does it mean the hydraulic fluid is colored (useful in case of leak)? Does it means the elements of each circuit is painted accordingly (usefull for maintenance)?

Only some manufacturers label hydraulic systems by colours. Airbus names them Green, Yellow and Blue, but Boeing usually names them A, B and Standby.

and at least some of the Dassault aircraft number the systems (1, 2, etc.)...

A common hydraulic fluid used in aircraft is called Skydrol. The fluid is artificially colored, but this does not depend on which circuit it is in. The FAQ on the manufacturer's site addresses the color.

Skydrol fluids are given a purple dye to make it easy to distinguish them from other fluids. But the dye does not usually last as long as the fluid does. The color may change from purple to gray or yellow or green, yet the fluid can still meet all used fluid specifications. Color is not a reliable indicator of fluid quality, and we recommend that the system be sampled for chemical analysis to determine if it is suitable for use. A dark amber color, however, is a cause for concern, and can indicate a severely stressed fluid. Call Solutia's Technical Service for assistance if you are unsure about a fluid's color. Our technical bulletin has a color chart to use as a guide.

Initially the color helps to distinguish from other fluids, and having multiple colors would make it more complicated. It notes that the color also does not remain constant, so even if it were colored initially, it would probably end up changing anyway. It also mentions that a dark amber color can be an indicator of issues.
I can't find any good pictures, but the lines are probably not color coded. Any colored hydraulic system components are generally purple (like the fluid). The lines themselves can be labeled, but the standard for hydraulic lines is blue and yellow. The end fittings are colored to identify the tube material type.
The maintenance manuals will show the layout and label the different lines. Also, on this ASRS page, one report is from a mechanic that confused the green and yellow system fittings, which probably wouldn't have happened if they were color coded.

It is just the components that will be labeled accordingly.
Having each fluid be colored differently is a maintenance nightmare, you need to have all 3 colors available in the hangar and each system must be fully isolated (3 different pumps, buffers, actuators etc.) to avoid contamination.

That would actually be an excellent idea. The plane's systems are separate, maintenance can just stock the clear fluid and some jars of dye. Hydraulic fluid is naturally a clear oil, the common red colour is already an additive, and a top-up wouldn't need any dye added (just mixes in with the rest).

top-up would dilute the dye, and wouldn't solve the weight issue for the fully separate systems

I think this is a good idea to use colors as labels. This could have been used to visually identify either the fluid itself or the several components (pumps, actuators, pipes)

@ratchetfreak a fluid top-up won't make red/ green/ blue dye fade out to an indistinguishable colour. If it does, then it's more than a top-up.

Having 3 different fluids in maintenance may be inconvenient, but it does eliminate a common failure mode (contamination at the source).

@paul: And if someone puts red hydraulic fluid in the blue system by mistake?

The colour codes of hydraulic systems referred to, common for 1960's-1970's British Jets, and since then all Airbus aircraft, do not refer to the colour of the fluid!
It is just an identification system for each separate hydraulic branch, the fluids themselves have nothing to do with 'green, yellow, blue' system identifications.
It should also be noted that the American convention is not to use colour codes, on Boeings they typically refer to 'left, right and center' systems.
In terms of the fluid itself, the original question mentioned Concorde. Concorde used Chevron M2V 'Oronite' fluid, which is silicate ester and is a light straw colour.
Almost all civilian airliners used one of the main fire-resistant phosphate ester fluids, either Skydrol or Hyjet - phosphate ester is also naturally a light straw colour, but as specified it is artificially dyed purple for identification. However, phosphate ester aviation hydraulic fluid changes colour when overheated, getting darker, turning from purple to dark purple to grey, to black in extreme cases - or if it leaks and becomes oxidised it turns back to the normal straw yellow colour.
Final mention is for many business jets(Gulfstreams and Citation X excepted) and military aircraft, they use 'red oil' hydraulic fluids, either Mil-5606, 87257 or 83282. They again are probably a straw colour to begin with, but are artificially dyed red per their relevant specifications.
You can generally mix the different red oils, or you can mix the different phosphate esters, but never ever mix red oil and phosphate ester - they have different seal swells etc.
Getting back to the original question: you don't use fluid colour to identify the system, you use the system schematics and the tube/hose identification labels. The colours 'blue/green/yellow' are primarily for illustrative purposes in the pilot operating manual and system descriptions - go to smart cockpit dot com for exmaples.

Your answer is quite good and may be enhanced with references and links for further readings.


text:
How were large pieces of the A300-600ST and the B747LCF transported?
I will present my reflection based on the Airbus A300-600ST Beluga and I suppose the reflection is the same with the Boeing B747LCF Dreamlifter:
The A300-600ST was built because its predecessor (the B377SGT Supper Guppy) was not large enough to transport future aircraft's large pieces. Those pieces are too large to be transported by train or by road.
But the A300-600ST's pieces must have been transported to build the aircraft. Some of the A300-600ST's pieces must be larger than the pieces this aircraft is designed to transport. So I'm puzzled because I cannot figure out how large pieces of this heavy transporter were transported to its assembly site. The same reflection goes with the B747LCF.

The reason to have the Beluga and the Dreamlifter is to bring sub-assemblies to the final assembly line. But if all sub-assemblies are built in one place there is no need to transport large components. I assume the Belugas were fully built and assembled in Toulouse. The LCF were built by Evergreen in Taiwan. The main sub-assembly came there on its own wings, the rest was manufactured on site.

@DeltaLima OK for the LCF. But for the Beluga, as you can see in the responses, large pieces were built by CASA which wass located in Spain. You may assume it was fully built AND assembled in Toulouse, but I cannot find any evidence of this.

Nothing is 'too large to be transported by train or by road'. The transport just becomes costly (lots of temporary changes along the route) and complicated to plan.

The answer may be that parts were transported the same way A380 parts were transported

The A380’s size means its fuselage and wing sections are shipped via a surface transportation network that includes specially-commissioned roll-on roll-off ships to carry these sections from production sites in France, Germany, Spain and the United Kingdom to the French city of Bordeaux. From there, sections are transported by barge along the Garonne River to the Toulouse final assembly line.

From Airbus - TRANSPORT OF MAJOR AIRCRAFT SECTIONS
According to All about Guppys

The A300-600ST was built in Toulouse, France.

All the smaller pieces would have fitted into an existing transport aircraft.

I think the answer is more complicated: (i) "built in Toulouse" does not mean some part have been built elsewhere, transported to Toulouse and then assembled at Toulouse (I think of the parts from CASA and Aerostructures in your illustration); (ii) the A380's transportation network was built (in 2004) after the first A300-600ST assembly (1994).

@ManuH: Note that the first few years of A300 production, before Airbus acquired the Super Guppies, were also via road and rail; presumably the outsize Beluga parts were transported in the same way.

The A380 road transport network is not the only way to move outsize loads on road. For the A380 those transports are regular enough to warrant making changes to the roads. Non-regular outsize transports require more planning and ad-hoc changes (moving obstacles out of the way temporarily, then reinstalling the obstacle after the transport passes). This is done routinely for all kinds of outsize loads.

this video confirms road transport for Beluga XL: https://www.youtube.com/watch?v=f7lGT4S9QnA

@Hobbes now that routes have been adapted for the A380, it is quite easier to find a suitable road to transport large pieces over thousands of kilometers. At the A300-600ST time, such road didn't exist.

You absolutely could transport large pieces via road to Toulouse in the 1970s, you just caused more disruption (road closures for a day or two, temporary removal of traffic signs, traffic lights, etc) than you would today.

The A300-600ST aircraft were built new. The sections were built by companies within the consortium, and probably shipped similarly to other Airbus parts to the final assembly location.
The 747 LCF aircraft were not built new. The aircraft were acquired second hand from airlines, and sent to Taiwan for the modifications. The modified fuselage sections could have been shipped individually and assembled on site. Shipping options are similar to those of other large aerospace parts and include sea, rail, and air.

The sea option used by the A380 to go convey large pieces to Toulouse was not available at that time (the part going from the see to Toulouse was built on 2004 as mentionned in other responses and comments), air transportation was problematic as exposed in the question and  rail is more problematic as the A300-600ST is built pieces were too large for rail transportation.

@ManuH: That's why we have really big trucks!


text:
In an international airport, is there specific set of laws for the international area?
When travelling in an international flight, you pass a border with passport check at both the departure and the arrival airports. It means you spend time in an international area. What are the applicable laws in this area? The one from the country the airport is located in? A special set of international laws?
Are you juridically out of any country (as if in international waters)? Is there special status with flights between countries with special immigration agreements (e.g. Shengen area)?

You might try asking this in Travel.se, they discuss transit areas of airports quite often.

@CGCampbell OK.  [DONE].


text:
Why is "runway behind you" useless?
I was recently answering this question and for some reason it made me think of this fairly famous aviation saying

The three most useless things to a pilot are the altitude above you, runway behind you and fuel on the ground

Now, I get altitude above you - you may as well use some of it lest you meet a mountain. I also get fuel on the ground - if in any doubt fuel up before you go.
But I'm not sure I understand why "runway behind you" is useless in this context
Could someone explain?

*sigh* Please people, stop asking how to use a runway behind you. For what its worth I was interested if this was a literal saying or if it was figuratively speaking about something other than the obvious answer.

Altitude below you is useful for lots more things than "not flying into a mountain".  The main use is if you are at a nice high altitude AGL and a problem develops, you have time to think about what to do next, and possibly discuss the issue with co-pilot/ATC and then formulate a plan.

The saying is a bit confusing because 2 of the 3 examples are clearly referring to flying at altitude and the "runway" one only applies to being in the process of takeoff or landing.  A "runway behind you" can be very useful if you are high enough to turn around and land on it, especially if it's the closest one in an emergency

The runway ahead of you is useful to accelerate and - if needed - to decelerate.
If you enter (or touch down) the runway in the middle, you will not be able to benefit from the part of the runway behind your entering point. It is the same as if that part would not exist. Therefore it is useless.

unless the runway behind you has arrestor wires on it.

... and you caught one... @Sirex

@Harper-ReinstateMonica or you needed to not catch one (not equipped).  Unlikely but I was reminded of this famous incident

@Sirex Even then, it's not the runway itself that is useful, but the wires.

It refers to takeoff from an intersection rather than using the full runway length. The part of the runway behind you is now useless. It is part of the saying because the TODA (takeoff distance available) is now reduced and this needs to be taken into account when doing performance calculations. It is e.g. discussed in this thread on pprune.org.
One could also interpret it to refer to landing: When touching down half way down the runway, the part behind you is useless for stopping.

OTOH, if you happen to be departing an airport with an 11,000 ft runway in say a Piper Cherokee, most of that runway is pretty useless to you :-)

Not if you lose an engine 20' off the deck.

@Michael Hall: A good bit of it is, 'cause I can sure land in a lot less than 9000 ft from 20 ft AGL.

Yes, but there is a big difference between useless, and not necessary.

@jamesqf If you lose an engine at 200', you'll still be thankful for that 11,000' runway. :)

In case of engine failure during the takeoff run, it might be possible to land immediately and stop if there's enough runway. But if the run was started partway down the runway, there will be less room to do that, possibly resulting in a more severe result.
The point of all three "useless" items is that, by using them before a problem arises, the results of the problem may be greatly mitigated. It's an indirect way of recommending making use of the resources available to be prepared.

All three of the useless data are contrasts with useful information:

the altitude above you

What matters is how much air is below us.

runway behind you

We're only interested in how much runway is available in front of us.

fuel on the ground

We care about how much fuel is in our tanks.

I've always known the saying as "The three most useless things to a pilot are the altitude above you, runway behind you, and the fuel you had"
What the saying is really talking about, is give up your margin grudgingly, especially when you don't need to.
This works for all aspects of life.


text:
Why do cabin crew also man check-in desks?
When flying, I sometimes notice that the person behind the check-in desk also turns out to be the flight attendant on my flight.

Is this always the case, or did I just notice the exceptions?
Why do airports use cabin crew for the (comparatively) simple job of checking bags, which could in theory be done by (presumably) cheaper lower-skilled employees? Or am I underestimating the difficulty of checking bags?

When margin is everything, why employ 2 people, when you can employ one to do both jobs? The flight attendant is not busy during checkin, and the checkin person is not busy during the pre-flight.

All I hope is not to see one of the pilots behind that check-in desk some day - you never know what this economy can do to you.

What airline was this?

@woakley5: Air Berlin.

@RaajTram Try small regional carries in places like the Caribbean.  On one flight, the captain and only pilot (I sat on the right), checked me in, carried my bags to the aircraft and boarded me.  It wouldn't have surprised my if he was the mechanic, the refueller and the CEO.  It was a cool flight.

@Simon: I would think twice before boarding such kind of flights where pilots are stressed with these kind of duties.. There have been crashes in these kind of scenarios

@Firee The taxi driver regularly double as mechanic, carry the luggage, take phone reservation, ...
There have been accidents in these kind of scenarios.

It probably depends on the airline and airport, because on the flights I have been on, the checking-in is managed by "check-in staff" who are not flight attendants. Larger airlines do the arrangement that I just describe while smaller ones might manage everything with the flight crew.

This mostly like because the airlines you took may not officially operate in that country. They only fly there. So the airport doesn't hire staff to do the check-ins and leave it to airlines to figure how they want to do it.

the airport and the airline are two different companies. the fact that it is not a hub for the airline has no bearing on which people the airport hires (as it may be the hub of another airline, for example)

Maybe it is different depending on the airline but airlines typically employee Gate Agents to check passengers in and get the passengers from the boarding area to the aircraft. You will typically not meet the Flight Attendants until you are boarding the aircraft as they have a list of duties they must accomplish before the passengers begin boarding as well as while the passengers are boarding the aircraft. These Gate Agents can sometimes be easily mistaken for Flight Attendants because their uniforms sometimes appear similar.

And sometimes FAs with no flying duties (maybe they're temporarilly grounded for medical reasons) are assigned other duties like manning checkin desks until they can go back on flight status. But the same people doing checkin as being on the flight? Never saw that.


text:
Why are there separate certification rules for aircraft and engines?
I have a question about certification rules for large civil aircraft. I know that these are certified according to (for example) EASA CS25. Additionally, there are separate certification rules for engines (for example, EASA CSE).
Why does this distinction exist? I would have said that engines are an integral part of aircraft, but perhaps I'm wrong here?

Yes, engines are an integral part of the aircraft. But they are also very complex items. They are typically designed and manufactured by someone other than the airframe manufacturer. the engine manufacturer is responsible for making sure the engines perform their intended function reliably and safely.
Most of the engine functional requirements are engine specific and independent of the aircraft it is installed on. These requirements are covered by the engine type certificate (TC). The engine TC provides evidence that the engine meets these requirements.
When the engine is installed on the aircraft, the aircraft manufacturer has to show compliance to all the rules including the engine requirements. The engine TC shows that the engine complies with the engine specific requirements. The aircraft manufacturer then only has to show that the installation of the engine(s) comply with the aircraft requirements.
This means that the aircraft manufacturer doesn't have to deal with engine design details it may not have full understanding of. It also reduces duplication of effort when the engine is used on another aircraft. Without the engine TC, every new aircraft that uses the engine would have to go through the engine certification details.
It is also tied to production certificates (PC). The engine manufacturer has to have a PC to build the engines. Getting the PC involves proving that you can manufacture it to conform to the TC. By owning the TC, they can control the design and the production. If the engine was 'just an aircraft part' it would be part of the aircraft TC and the design owned by the aircraft manufacturer. In this case, the engines would be built under a parts manufacturing authorization (PMA) and each engine built would be tied to a specific aircraft type. That's not very efficient.

"They are typically designed and manufactured by someone other than the airframe manufacturer." Are there any airframe manufacturers that also build engines? At least, any still in existence...

@FreeMan Technically Lycoming and Cessna are both part of Textron Corporation, but they operate as independent companies. In the past, UTC owned P&W and Sikorsky. Considering the number of mergers, acquisitions, and divestitures in the industry I would hesitate to to claim an absolute that could change in the future.

Fair point, sir!


text:
What is the purpose of the 787 engines extending on landing?
As seen about 53 seconds into this video of the Qantas Dreamliner landing, the engine housings seem to extend upon landing. What specifically is the purpose of this? To balance? To help with air flow?

Note that you can include a timestamp in your video link by right-clicking on the timeline "cursor" in the youtube video and selecting "copy url including timestamp" or something like that.

Those are the thrust reversers, which cause the engines to produce thrust "backwards" to help slow the aircraft during the landing.
You can see a close-up video of 787 thrust reversers in operation, including a look at the panels at the back of the engine which block the normal flow of air.
We also have a question How do thrust reversers work on a jet engine, and when should they be used?, which describes their operation in general.

And they're not unique to the 787.  Most if not all commercial airliners have them.

And the loud roar you hear just after touchdown every time is exactly that!

@HankyPanky The spoilers also make a lot of noise, even when reversers aren't used.

those are the thrust reversers. when they open up, the fan air flow is diverted and discharged forward instead of aft. this helps slow down the plane and lets the wheel brakes last longer.

Most of the time the aircraft uses the aircraft brakes, they provide far more breaking power than the reversers. Most airports have noise restrictions and thus only idle reverse thrust is used. Modern carbon brakes last a set of applications where brakes force is no factor. Older steel brakes wear out earlier if there is larger force applied.


text:
Do Jet-A and 100LL smell different?
Recently, a small plane operating from a nearby airport suffered an engine failure on takeoff and crashed.  One of the hazmat responders dealing with the resulting fuel spill mentioned "a strong smell of Jet A", leading to speculation that the airplane had its tanks filled with the wrong fuel.  Would he have been able to distinguish Jet A from 100LL by smell?

Guys who spend all day fueling aircraft know the difference.

If you know what you're looking (sniffing) for, yes.
Avgas 100LL smells like gasoline (a little different than what you pump into your car, but similar).
Jet-A smells like kerosene (not quite "like diesel", but definitely not like gasoline).
If you are familiar with how both smell you can tell them apart by odor, and a significant contamination of Avgas with Jet-A could be detected by smell. (Other tests, like an evaporation test, are more reliable for lower levels of contamination however).

Note that this kind of sniff test is like how if you've worked with solvents you can tell alcohol, acetone, and MEK apart by smell - it may be a useful talent occasionally, but if you make a habit of doing it to impress people at parties or something you'll probably run out of brain cells in a hurry.

It would also be a useful talent when filling up your plane, so that the first responders don't have to diagnose that for you.

@fooot Presumably the BIG SIGNS on the fuel truck and pump (and the fact that the Jet-A nozzle shouldn't fit into an avgas tank opening) would help there too -- but yet witness the thankfully-infrequent misfueling errors still happening…

@voretaq7 - assuming, of course, that the fuel in the tank matches the sign -- my dad (and a few other people) once got an engine rebuilt for "free" when a gas station pumped diesel into his tank instead of gasoline, apparently the delivery truck put the diesel in the wrong tank. The car left an impressive smoke trail before it died. I don't know if this happens much (if at all) at airports, but it does happen at gas stations from time to time

@Johnny Happens at airports too - somehow Jet-A gets pumped into the Avgas truck, and then the truck's pumping system needs to be taken to bits to clean it out and all the filters have to be changed. (Presumably the line person who makes such a mistake is thrown into the Jet-A tank in the fuel farm to drown as punishment for the work they created).

Shouldn't it be also possible to tell by colour? AvGas is dyed. Not sure whether Jet-A is, but they sure end up with different colours.

@JanHudec If it's pure Avgas or pure Jet-A yes: 100LL will be a light blue color, and Jet-A is undyed (usually described as "straw yellow"). If the two are mixed together though you can have a mix that would destroy a piston engine but still look "blue" in the fuel sample cup -- that's where the telltale odor of Jet-A (or an evaporation check) will catch the problem for you.

Sounds like there was no fire at the accident site so that's good.
It is possible that a mixture of 100LL and JetA (say 3/4 tank of 100LL topped off with JetA) could have passed the smell test and color test during sumping but still have caused the accident.
Possibly the Avgas evaporated quickly at the scene leaving only the jet fuel lingering for the hazmat responder to smell. So even a partial mixture could have done it.
However, if you pulled up to any arbitrary FBO or self service rig to fuel your aircraft, you would not be able to get the JetA nozzle to fit in your tank. The following is an excerpt from this wiki page: http://en.wikipedia.org/wiki/Aviation_fuel

Because of the danger of confusing the fuel types, precautions are
taken to distinguish between avgas and jet fuel beyond clearly marking
all containers, vehicles, and piping. The aperture on fuel tanks of
aircraft requiring avgas cannot be greater than 60 millimetres in
diameter. Avgas is often dyed and is dispensed from nozzles with a
diameter of 40 mm (49 mm in the USA).[7] [8]
Jet fuel is clear to straw-colored, and is dispensed from a special
nozzle called a J spout that has a rectangular opening larger than 60
mm diagonally, so as not to fit into avgas ports. However, some jet
and turbine aircraft, such as some models of the Astar helicopter,
have a fueling port too small for the J spout, and thus require a
smaller nozzle.

Here is a graphic showing the two spout sizes:

Simply put, a JetA fueling nozzle will not fit in the aircraft tank's filler neck. So if there was a fueling mishap, it almost needed to be done by the operator with a homebuilt self fueling rig in his hangar.
Side note: In the late 80s or early 90s when I owned an FBO with a maintenance shop, the Chevron facility in Richmond CA had a refinery valve mishap and filled several tanker trucks with avgas while JetA was leaking into the delivery apparatus. There were many dozens of piston aircraft fueled by the time they discovered and remedied it. The FAA issued an AD against all aircraft that got some of this fuel and Chevron bought each owner an overhaul.

I once read of an accident caused by the fuel truck or tank itself being filled with the wrong fuel. There was another accident caused by the fuel trucks not having the proper nozzles. If I can locate the accidents, I will post them. Either way, there was a failure or break down in accepted safety procedures.


text:
How to check if (small) aircraft's radio is working properly?
Is there a standard way to test/check aircraft's radio without calling anybody (Ground/Tower) to see if it is receiving and transmitting?
Is there a way to check aircraft radio with your own handheld radio on the ground before takeoff? I am referring to small aircraft type like Cessna 172.

The easiest way to do it is to tune to an ATIS/AWOS frequency and make sure you can receive clearly. Many radios also have a way to override the automatic squelch to make sure you can adjust your receive volume correctly. Something like this video: http://www.youtube.com/watch?v=9LWpnGjQCis&t=5m0s
As far as checking if you can Transmit, its kind of hard since you need someone to respond to you. Its pretty common to hear pilots on tower/ground frequencies asking for a mic check. Not sure if this is standard phraseology but it gets the point across

How do you check if you are transmitting fine?

Why would you not want to contact ground/tower? You could also use a hand-held radio and tune to an unused frequency...

I agree with Ron. Why not contact ground/Tower? If you're at a controlled field, you're required to talk to them anyway if you actually want to go somewhere.

If you are going to use a handheld, it’s best to use the Unicom frequency for the airport. That’s what it is for. Or if it is busy, use the air-to-air frequency reserved for fixed-wing aircraft—122.75 (in the US).

Here are some things you can do, there are probably more. I'm assuming you're mainly interested in checking COM, not NAV.
Controlled airport
Check if you can hear the ATIS, Ground or Tower; if you can, then at least you know you can receive. But you might as well just call Ground immediately and ask for a radio check. The correct phraseology in the US (see the Pilot/Controller Glossary) is "how do you hear me?":

Bowman Ground, N12345, radio check please. How do you hear me?

Uncontrolled airport
Listen to the AWOS, if there is one, to determine if you can receive. Ask UNICOM or another aircraft to give you a radio check. If none of those options are available then you can use a handheld airband transceiver to test. Tune it to the frequency you're listening on and click transmit a few times, as if you're turning on pilot-controlled lights (PCL). If you hear the clicks in your headset, you can receive. To test transmission, do the opposite: click the transmit button on the yoke and listen on the handheld.
Airborne
You could call Approach, Center or FSS; try activating the nearest PCL (works best at night); tune and identify a navaid (that doesn't verify COM works, but at least you know something's working); listen to 121.5 (in the US, you should be doing that anyway) because there's usually some bored pilot asking for sports scores on it and some other equally bored pilot yelling at him to "stop talking on guard!".
Troubleshooting
As always, if you have any problems with transmitting or receiving do some basic troubleshooting first: check you have the correct radio selected; check you have the correct frequency; check the volume controls on your headset and/or panel; turn the squelch up/down/off; check your headset is plugged in correctly; if you have a right-seat passenger, see if they can hear/transmit; and anything else you can think of.

Clicking can also confirm transmission with pilot controlled lighting or when activating a SuperAWOS.

the link to correct phraseology is gone

@vitaly Thanks, the FAA has broken its links yet again.

@JWalters not sure how likely such issues are, but that wouldn't work to find problems where your PTT is working but audio isn't and your radio only transmits a carrier.

Just call clearance or ground or tower or CTAF and ask "Radio Check".  They will acknowledge you if they hear you.

I mentioned in my question that without calling ground or tower.


text:
As a passenger, can I use my handheld radio in commercial flights?
During flight from one country to another, can I use my hand held radio during flight to listen to ATC?

This is receiving-only, correct?

yes, just to listen to the ATC, not to interrupt or transmit.

Related: Can I fly with an airband receiver in the United States?

This actually varies from country to country. You can use it in the US (but I cant promise people wont look at you funny). In the UK its actually illegal to do so you can see a full discussion here on that topic. If you intend on doing this I would advise checking the local laws on what you can and cant do.

It will also depend on airline policy: If the cabin crew tells you no then the answer is no.

To say "you can use it in the US" is too broad.  It depends on the airline.  Radio receivers have the potential to fail in a way that causes transmissions.  As an example, United prohibits radio receivers.  https://www.united.com/web/en-US/content/travel/inflight/devices.aspx

I found it for Emirates and it mentioned that you cannot use it at all. http://www.emirates.com/english/help/faq/193449/what-personal-electronic-devices-can-i-use-on-board

I think you need to update your answer to "check Airline specific policy", instead of "local laws"

I was speaking mainly from an FCC (legal) stand point. While airlines may not allow it all they will do is ask you to shut if off. In the UK it seems you can get in real legal trouble for it.

I don't know if you are "authorized" to... but I can tell you that it likely won't work.
I've tried a handheld aviation radio in a commercial cabin, and was unable to hear the ATC side of transmissions at all.  This was likely due to having a very short antenna inside a large metal cylinder, a long way from the source of the transmission.
It was nearly impossible to follow the communications when I could only hear my flight crew's side of the conversation.


text:
What type of commercial aircrafts generate strong Wake Turbulence?
I know that 757, 737 generate strong Wake Turbulence and as pilots, we should always be aware of that and caution for the wake turbulence while landing and departing.
I would like to know if is there like a list of commercial aircraft based on their Wake turbulence generation?  Which one generate the most and which one generate the least? Let's assume they are all heavy, clean, slow!

I don't believe that the 737 is especially unusual but the 757 is often classified as "heavy" even though it's weight does not compare to a 747 for example.  It's supercritical wing can generate wake vortices that exceed those of the 747 in some flight phases.

Big ones.
Wake turbulence is primarily proportional to the aircraft's weight, since this is generally directly related to the size of the aircraft and the amount of air the wings need to move to provide enough lift. Aircraft are categorized with respect to wake turbulence based on their maximum certificated takeoff weights. FAA AC 90-23G describes classes for ATC purposes as follows:

Super - Interim class for the A380 and AN225
Heavy - 300,000 lb or greater
Large - 41,000 - 300,000 lb
Small - 41,000 lb or less

Heavy and Super category aircraft are required to add their category to their callsign when contacting ATC, e.g., "Speedbird 12 Super".
Aircraft in higher categories have larger separation requirements when in front on aircraft in smaller categories, such as between takeoffs or landings, and ATC will issue caution messages cautionary advisories when required (Section 2-1-20).
The AC linked above discusses further on wake turbulence, such as how it propagates and how it should be avoided or escaped.
The FAA is also working on a 6 category system (A to F), which is in effect at MEM, SDF, CVG, and ATL. There are plans to expand this to the Metroplexes of Houston, New York, Charlotte, Chicago, and Northern California in 2015.

This approval was based on years of joint research and development by
  the FAA, Eurocontrol, scientific experts in wake, and experts in
  safety and risk analysis. Categories are now based on weight,
  certificated approach speeds, wing characteristics, along with special
  consideration given to aircraft with limited ability to counteract
  adverse rolls.


text:
Why not use flaps in take off from High Density Altitude airport?
Why can we use flaps in short field take off but not in high density altitude airports?
Why can't they be used in high density altitude to shorten the take off distance as in short field take off?
Why short field take off with flaps doesn't apply to high density altitude airports?

With more flaps, you will lift off at lower speed, but will have worse climb performance due to the increased drag.
On short field you are limited by the field length, so you need to lift off early and therefore need to use more flaps.
But in the mountains you are limited by the climb performance. Therefore you need to use less flaps.
Of course, this is just general rule. You should look up performance for given density altitude and compare it with available runway length and required climb gradient.
Update: Extending flaps never improves rate of climb. Up to some points it does improve angle of climb though, because it reduces Vx and the engine can produce more thrust at lower speed. The setting for best angle of climb is higher than for normal take-off and is used on obstructed field.
However as excess power reduces (with density altitude), Vx increases towards Vy (at absolute ceiling, Vx = Vy and rate of climb is 0) and the flap setting for best angle of climb decreases. So even for best angle of climb you shouldn't use as much flaps at high density altitude as at low.
And most mountain airports are not obstructed. In most cases the valley ahead is large enough, but you want to get above the tricky winds fast and you want high rate of climb in case you encounter a downdraft. And that means no or little flaps.

I was taught to use same indicated air speed to rotate in short  field take off and normal take off.  Are you talking about true speed here?

@PMoubed, no, indicated. And at low altitudes they are the same anyway. What you say is strange, because you achieve the same speed after the same distance—and if the field is short, you don't have that distance. On the other hand if the field is obstructed (runway is long, but then you need to climb steeply), this actually advocates rotating at higher speed (if paved, accelerating in ground effect if not).

Some wing designs achieve best lift:drag with some flap deflection rather than flaps0. Flaps0 is still used for cruise because of reduced total drag at the lower lift coefficients needed for cruise. Thus best climb rate and angle (and ceiling) can be had with some use of flaps. However this is highly dependent on the design and it is possible the gains may be too small to balance the extra operational complication. Especially when including the variables of  zero AoA ground roll, ground effect, and angle of climb vs point of rotation, for a 50ft obstacle. So POH settings are based on testing.

A cursory glance of this site reveals that the slight increase in lift is not worth the larger increase in drag when deploying flaps at high-altitude/low air-density conditions.
http://www.newconthenet.com/mountainflying.html

Since the list gives, as a second point, a rule about X% of take-off speed at Y% of runway length, while as explained here no such rule can be given with any generality, I would not trust it too much. Many of the rules are right, but the performance ones tend to be over-generalized—your performance calculation might yield different result.

It depends on how much excess horsepower you are packing.
Large passenger jets can and do deploy high-lift devices before takeoff because they are more likely to have enough power to overcome the drag penalty in the interests of increasing lift.
Small planes can't do exactly the same because they are more likely to be power-limited, and generally they have to take off in a low-drag configuration- meaning 10 degrees or less flaps on takeoff.

I'm sure I recall seeing at least 10% flaps recommended in some cases for takeoff in flight manual for Ces. 152.  Just saying something is "power-limited" is too absolutist-- everything in aviation is affected by the fact that power is not infinite.  It's just a matter of where on the curve the data point happens to fall, as to whether or not it is worth deploying flaps etc.

Here we go: "Using 10° wing flaps reduces the total
distance over an obstacle by approximately 10%." and "On a short field, 10° wing flaps and an obstacle clearance speed of 54 KIAS should be used. This speed
provides the best overall climb speed to clear obstacles when taking into account turbulence often
found near ground level."

This is not a question that can be answered with a general rule.
What must be examined, when taking off in a condition of lower atmospheric density, is the altitude limit for that weight.
The issue is: can the engine/prop generate enough thrust to fly?
The best hope is at the speed where prop efficiency is the highest.

is this Vmin power?

Well, no, because at full RPM the prop spins faster, so it needs a faster forward speed to keep relative wind at the best angle of attack.

what is that angle of attack?

Well, why not put a Clark Y on your wing and your propeller?
The magnificent Clark Y maintains optimum Lift/Drag efficiency from about 3 to 7 degrees, making it ideal for fixed pitch prop applications and wings for GA aircraft.
One can then evaluate the effects of lowering flaps even 10 degrees on lift and drag:  it shortens gliding distance.  In other words, it's L/D ratio is less.
At high altitudes, where thrust is limited, it is best to fly in the "sweet spot" where Lift/Drag ratios are optimum for prop and wing.
This is where climbing performance, so important at higher altitudes is best.
Flaps may get one off the ground faster, only to provide a nasty surprise, once trying to climb out of ground effect, if power is insufficient.


text:
Should 3 takeoffs and landings to stay day current be solo?
Let's assume  that I fly every weekend with a friend which is not a pilot. Can I log that time as PIC and count the takeoffs and landings toward the currency requirement as stated below:

Sec. 61.57: Except as provided in paragraph (e) of this section, no person may act
  as a pilot in command of an aircraft carrying passengers or of an
  aircraft certificated for more than one pilot flight crewmember unless
  that person has made at least three takeoffs and three landings within
  the preceding 90 days, and -- The person acted as the sole manipulator
  of the flight controls; and The required takeoffs and landings were
  performed in an aircraft of the same category, class, and type (if a
  type rating is required), and, if the aircraft to be flown is an
  airplane with a tailwheel, the takeoffs and landings must have been
  made to a full stop in an airplane with a tailwheel.

Does sole manipulator of the flight controls means solo and by yourself (alone) in the airplane such as Cessna-172 or does it mean you can have a non-pilot passengers?

The term solo is used when they want you to be the only one in the aircraft.

I'd certainly hope that being solo isn't required... in over (mumble, mumble) hours carrying passengers, I haven't logged a moment of solo time yet, but the airline seems to accept that I've stayed current  by getting my 3-3-3 every 90 days.

Yes you can count that time as PIC and towards this currency requirement.
The regulation requires that you are the only one who moved the flight controls during takeoff and landing. This means you could fly with anyone else in the cockpit (including a flight instructor, another pilot, or non-pilot friends) but you alone must be the person who performs those takeoffs and landings in order to remain qualified.
This eliminates a few scenarios from counting towards the currency requirement, including:

You flew with another pilot, counted yourself as PIC (because you were more qualified or by prior arrangement) but didn’t actually do the work yourself
You flew with a passenger, and the passenger manipulated the controls during takeoff and landing
You flew with an instructor, and they helped you with the landings

On the other hand it should include the case when you are pilot flying, but not PIC, right? Because that's how you restore currency if you somehow lost it: by flying with another pilot that acts as PIC while you are PF.

@JanHudec -- yes, as you were the only one who did PF duties.

@Jan Hudec - If you lose currency for day VFR, you simply have to log 3 takeoff / landings while solo to regain currency. You do not need another pilot, you simply can't carry a passenger until you get your currency back.
If you didn't have your currency, you could go to the airport with a friend, get in the plane by yourself, do your 3 landings, pick up your friend and go fly.

@gregtaylor, you do not have to be solo for the 3 in last 90, just the sole manipulator of the controls. Another pilot rated for that airplane who is passenger-current can also come to act as PIC. They can't log PIC time, as the situation of flight does not require 2 crew, but they are taking the responsibility for the flight and are qualified to take you as a "passenger", even though you'll be on the controls.  61.57(a) specifies 3 in 90 to act as PIC.
https://aviation.stackexchange.com/questions/27064/can-i-bring-a-person-with-me-while-getting-landing-current-if-theyre-a-licensed

The other answers are correct. The regulations do not require you to be solo in order to log PIC time or the landings for the currency. The only problem would arise would be if you are already outside of the 90 day currency window. At that point you would be required to complete the takeoffs and landings solo (meaning you are the sole occupant of the aircraft) or with a flight instructor occupying the other seat. A flight instructor is not considered a "passenger" when they are acting in the capacity of a flight instructor. Once those three takeoffs and landings are complete AND logged you may then carry passengers again.

you do not have to be solo for the 3 in last 90, just the sole manipulator of the controls. Another pilot rated for that airplane who is passenger-current can also come to act as PIC. They can't log PIC time, as the situation of flight does not require 2 crew, but they are taking the responsibility for the flight and are qualified to take you as a "passenger", even though you'll be on the controls.  61.57(a) specifies 3 in 90 to act as PIC.
https://aviation.stackexchange.com/questions/27064/can-i-bring-a-person-with-me-while-getting-landing-current-if-theyre-a-licensed


text:
Why is Cessna 172 POH performance chart based on Pressure Altitude and Temperature instead of Density Altitude?
While calculating takeoff distance for Cessna 172, POH need to be used. POH charts has pressure altitude and temperature and then distance for those two known parameters.
Does this mean if we have pressure altitude and temperature then we don't have to worry about density altitude in our calculation? Why would we want to calculate density altitude if we only need to know pressure altitude and temperature?

Is density altitude not calculated from pressure altitude and temperature?

@TomMcW Yes, those are two of the primary variables in calculating density altitude. Depending on what you read, some formulas allow you to use only those two variables. Others also—more accurately—include humidity or dewpoint as a variable.

The thing with Cessna POH data is that if you analyze it, the pressure altitude and temperature do not seem to correspond very accurately to density altitude. If you try to convert the given pressure altitude and temperature data into density altitude, the results are non-proportional. In other words, it is hard to reverse engineer the data to develop a table or formula that is based instead on density altitude.

Does it mean in Cessna POH charts, they have considered what density altitude going to be for a given pressure altitude and temperature?

I would imagine a bigger issue for this is that it's easy to measure pressure and temperature but less so density. Even though you can calculate density it'd easy to measure the others.

Temperature changes engine cooling, in addition to changing the density of the combustion charge. you didn't specify the submodel/year, long ago it was much easier to get pressure altitude and temperature information, and using this info directly for the chart saves unit conversion problems.

I have analyzed this information before, and this is what the chart looks like with one line for each weight category if you convert the temperature and pressure altitudes to density altitudes and then chart that vs. the takeoff distance:

You can see that I have drawn a sort of best-fit line for this information.  I did not add humidity as a factor because the manufacturer's charts had no relevant data.
Clearly, the trend from the manufacturer's data, when converted to density altitude, does not appear perfectly linear if you follow the data points they jump up and then start lower again in chunks of similar density altitude.
The greatest variation here is about 400 feet from the highest point in any of those clusters to the lowest point in the same cluster. That would be the yellow line around 8,000 ft of density altitude.
To answer your question, maybe they didn't use density altitude in the POH because their data appears to be irregular.
With that small margin of error here, I am comfortable using these best-fit lines to quickly calculate my data because I wouldn't make a go descision for a takeoff with a calculated distance anywhere near being within 400 feet of acceptable runway length.

I almost wonder if my best-fit lines aren't more accurate to actual performance.


text:
What do you squawk when multiple conditions apply?
There are three standard transponder codes for exceptional situations: 7500 (hijacking), 7600 (radio failure), and 7700 (general emergency).  Which code do you use if multiple situations apply?  For example, if a hijacker smashes your co-pilot's head into the radio, do you squawk 7500 for the hijacker, 7600 for the smashed radio, or 7700 for your co-pilot's head injury?

Trying to decide if this is a serious question or a troll....

@JohnK, the example may be less than serious, but the question itself isn't.  For example, you could reasonably find yourself in a situation of "radio failure + minor emergency".

If I ever find myself in that situation, setting the appropriate squawk code will probably be fairly low on my list of priorities  I'd say 7500 > 7700 > 7600 but ultimately only the pilot (or one of the pilots) on the spot can decide and any of those will get ATC's attention. FWIW, none of the 9/11 flights squawked 7500, as far as I remember.

I'd use 7500 when hijacked, no matter what else happens,
7600 if only my radios are broken but I am continuing the flight normally, 7700 if I have a radio failure and am in an emergency. You could also consider squawking 7600 first until you are in proximity to where you want to land, then switch to 7700 to show ATC: please also roll the trucks for me.

This is such an insanely improbable scanario that no regulations exist, or indeed should exist, to cover it. As always, the PIC should do what he finds most appropriate.

@Mark the answers seem consistent, is there a reason that you feel either do not adequately answer your question?

There is a trail of canceled AC's here that all say "only available within the FAA network" that might cover the topic if you can get ahold of them.
I found a brief mentions of it in this AC relating to ADS-B and its integration with transponders.

Operational procedures regarding the transmission of the generic
  emergency code (i.e., 7700) in cases when the flightcrew actually
  selected a discrete emergency code (e.g., 7500, 7600);

According to that language you basically use 7700 unless its explicitly one of the other two cases. In your example I would say a hijacking supersedes all but as with many things its a case by case kind of things.
Ultimately the codes are there as somewhat of a backup to actual radio telephony and cover 3 important cases the way I see it. In an actual scenario you use the code that is most applicable to the situation as the pilot in command sees it.

7500: The aircraft has been hijacked, they may not know what I (the pilot) is fiddling with so Im sending you this code since I cant say anything on the radio
7600: My radios are down, I may not be able to send or receive transmissions. There may be nothing else wrong with the aircraft in which case radio out procedures prevail. The tower may need to get a light gun out. In IMC see this question.
7700: Somethings going on this is how Im telling ATC for what ever reason chatting on the radio is insufficient. As to what the advantages are see this question.

This ICAO document on reply codes states that

ATS authorities should establish the procedures for the allotment of SSR codes in conformity with Regional Air Navigation agreements, taking into account other users of the system.
2.1.4.2 The following Mode A codes shall be reserved for special purposes
2.1.4.2.1 Code 7700 ... {2}7600 ... {3}7500 ...

So what will happen when you use the code is somewhat up to local jurisdiction and you should know what will happen when you enter the code to help you (as the pilot in command) decide which one to use.

If in doubt, squawk 7700. Even in a lost comms situation, you might choose to squawk 7700, even though 7600 is more precise, if you, the PIC, believe that an emergency situation exists simply because of the lost comms themselves, or if you're unsure of the local jurisdictional conventions.

Regarding the USA:
First, remember that those codes are intended for situations normally considered emergencies.
If any apply, you likely have an emergency, and then you also have the decision-making freedom of the pilot in command during an emergency per CFR 14 part 91.3 meaning you can choose the one you consider to be the most pertinent.
If I were being hijacked and had a gun to my head and then my engine failed, I would squawk 7500 because I would want search and rescue to know that wherever my craft went down, there's likely someone holding us hostage.
If my radio is out in IFR and my engine fails, I would squawk 7700 so that they don't assume I just landed in a grass field intentionally in response to a failed radio.  I want them to understand that generally, there is a big problem with my flight before going off-radar.
If I'm hijacked and my radio goes out, I would choose to squawk 7500 because my bigger concern is the hijack, and most likely, authorities would assume that my lack of radio contact was due to the hijacking anyway.
So, the short version:
If you have an emergency you should squawk what you think is best to keep yourself and your passengers alive.